The present invention relates to coated silicon nitride based cutting tools and a method of making the same. By a special heat treatment prior to the deposition of the coating, an improved bonding strength of the coating is obtained.
Silicon nitride is a highly covalent compound with a number of interesting engineering properties. An adverse effect of the strong bonding is a low self diffusivity which is why the material cannot be consolidated by solid state sintering. Sintering additives such as Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 and MgO are used to form a liquid with the SiO.sub.2 which is always present on the surface of the Si.sub.3 N.sub.4 grains. The resulting material has a two-phase microstructure of silicon nitride grains embedded in an intergranular bonding phase, which is normally a glass. During sintering, the silicon nitride grains obtain an elongated shape which has a positive effect on the fracture toughness of the material.
Uncoated silicon nitride cutting tools are used in cutting or milling applications of the cast irons. Attempts have been made to improve the tool life of the silicon nitride insert by application of wear resistant coatings. Coated silicon nitride cutting tool inserts are known, e.g., by the Sarin et al. U.S. Pat. Nos. 4,440,547, 4,431,431, etc.
However, the coating adhesion is not always satisfactory.